Fix play queue index handling in SyncPlay

Three index bugs in PlayQueueManager, two of which leave
PlayingItemIndex out of bounds, making every subsequent Buffering/Ready
request throw and leaving the group unusable until it empties:

- RemoveFromPlaylist did not compensate for removed items preceding the
  playing item: removing the playing item together with earlier items
  could select the wrong item or crash with an out-of-bounds index.
- Next/Previous on an empty playlist with RepeatOne/RepeatAll reported
  success or set PlayingItemIndex to 0 on an empty list, crashing
  downstream in Group and corrupting the index.
- SetPlayingItemByIndex accepted an index equal to the playlist count
  (latent off-by-one, callers currently pre-validate).

1 files changed, 156 insertions, 0 deletions

diff --git a/tests/Jellyfin.Server.Implementations.Tests/SyncPlay/PlayQueueManagerTests.cs b/tests/Jellyfin.Server.Implementations.Tests/SyncPlay/PlayQueueManagerTests.cs
new file mode 100644
index 0000000000..32685556b2
--- /dev/null
+++ b/tests/Jellyfin.Server.Implementations.Tests/SyncPlay/PlayQueueManagerTests.cs
@@ -0,0 +1,156 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using MediaBrowser.Controller.SyncPlay.Queue;
+using MediaBrowser.Model.SyncPlay;
+using Xunit;
+
+namespace Jellyfin.Server.Implementations.Tests.SyncPlay;
+
+public class PlayQueueManagerTests
+{
+    private static PlayQueueManager CreateQueue(int itemCount)
+    {
+        var items = Enumerable.Range(0, itemCount).Select(_ => Guid.NewGuid()).ToList();
+        var queue = new PlayQueueManager();
+        queue.SetPlaylist(items);
+        return queue;
+    }
+
+    [Fact]
+    public void RemoveFromPlaylist_PlayingItemAndPrecedingItemRemoved_PicksPreviousItem()
+    {
+        var queue = CreateQueue(5);
+        queue.SetPlayingItemByIndex(3);
+
+        var playlist = queue.GetPlaylist();
+        var expectedItemId = playlist[2].ItemId;
+        var toRemove = new List<Guid> { playlist[0].PlaylistItemId, playlist[3].PlaylistItemId };
+        var playingItemRemoved = queue.RemoveFromPlaylist(toRemove);
+
+        Assert.True(playingItemRemoved);
+        Assert.Equal(3, queue.GetPlaylist().Count);
+        Assert.Equal(1, queue.PlayingItemIndex);
+        Assert.Equal(expectedItemId, queue.GetPlayingItemId());
+    }
+
+    [Fact]
+    public void RemoveFromPlaylist_PlayingItemAndAllPrecedingItemsRemoved_PicksFirstRemainingItem()
+    {
+        var queue = CreateQueue(3);
+        queue.SetPlayingItemByIndex(2);
+
+        var playlist = queue.GetPlaylist();
+        var expectedItemId = playlist[1].ItemId;
+        var toRemove = new List<Guid> { playlist[0].PlaylistItemId, playlist[2].PlaylistItemId };
+        var playingItemRemoved = queue.RemoveFromPlaylist(toRemove);
+
+        Assert.True(playingItemRemoved);
+        Assert.Single(queue.GetPlaylist());
+        Assert.Equal(0, queue.PlayingItemIndex);
+        Assert.Equal(expectedItemId, queue.GetPlayingItemId());
+    }
+
+    [Fact]
+    public void RemoveFromPlaylist_AllItemsRemoved_ResetsPlayingItem()
+    {
+        var queue = CreateQueue(2);
+        queue.SetPlayingItemByIndex(1);
+
+        var toRemove = queue.GetPlaylist().Select(item => item.PlaylistItemId).ToList();
+        var playingItemRemoved = queue.RemoveFromPlaylist(toRemove);
+
+        Assert.True(playingItemRemoved);
+        Assert.Empty(queue.GetPlaylist());
+        Assert.False(queue.IsItemPlaying());
+        Assert.Equal(Guid.Empty, queue.GetPlayingItemPlaylistId());
+    }
+
+    [Fact]
+    public void RemoveFromPlaylist_ShuffleMode_PicksPreviousItem()
+    {
+        var queue = CreateQueue(5);
+        queue.SetShuffleMode(GroupShuffleMode.Shuffle);
+        queue.SetPlayingItemByIndex(3);
+
+        var playlist = queue.GetPlaylist();
+        var expectedItemId = playlist[2].ItemId;
+        var toRemove = new List<Guid> { playlist[0].PlaylistItemId, playlist[3].PlaylistItemId };
+        var playingItemRemoved = queue.RemoveFromPlaylist(toRemove);
+
+        Assert.True(playingItemRemoved);
+        Assert.Equal(3, queue.GetPlaylist().Count);
+        Assert.Equal(1, queue.PlayingItemIndex);
+        Assert.Equal(expectedItemId, queue.GetPlayingItemId());
+    }
+
+    [Fact]
+    public void RemoveFromPlaylist_PlayingItemNotRemoved_RestoresPlayingItem()
+    {
+        var queue = CreateQueue(3);
+        queue.SetPlayingItemByIndex(2);
+
+        var playlist = queue.GetPlaylist();
+        var expectedItemId = playlist[2].ItemId;
+        var toRemove = new List<Guid> { playlist[0].PlaylistItemId };
+        var playingItemRemoved = queue.RemoveFromPlaylist(toRemove);
+
+        Assert.False(playingItemRemoved);
+        Assert.Equal(1, queue.PlayingItemIndex);
+        Assert.Equal(expectedItemId, queue.GetPlayingItemId());
+    }
+
+    [Theory]
+    [InlineData(GroupRepeatMode.RepeatNone)]
+    [InlineData(GroupRepeatMode.RepeatOne)]
+    [InlineData(GroupRepeatMode.RepeatAll)]
+    public void Next_EmptyPlaylist_ReturnsFalse(GroupRepeatMode repeatMode)
+    {
+        var queue = new PlayQueueManager();
+        queue.SetRepeatMode(repeatMode);
+
+        Assert.False(queue.Next());
+        Assert.False(queue.IsItemPlaying());
+        Assert.Equal(Guid.Empty, queue.GetPlayingItemPlaylistId());
+    }
+
+    [Theory]
+    [InlineData(GroupRepeatMode.RepeatNone)]
+    [InlineData(GroupRepeatMode.RepeatOne)]
+    [InlineData(GroupRepeatMode.RepeatAll)]
+    public void Previous_EmptyPlaylist_ReturnsFalse(GroupRepeatMode repeatMode)
+    {
+        var queue = new PlayQueueManager();
+        queue.SetRepeatMode(repeatMode);
+
+        Assert.False(queue.Previous());
+        Assert.False(queue.IsItemPlaying());
+        Assert.Equal(Guid.Empty, queue.GetPlayingItemPlaylistId());
+    }
+
+    [Theory]
+    [InlineData(-1)]
+    [InlineData(2)]
+    [InlineData(3)]
+    public void SetPlayingItemByIndex_OutOfBounds_ResetsPlayingItem(int playlistIndex)
+    {
+        var queue = CreateQueue(2);
+
+        queue.SetPlayingItemByIndex(playlistIndex);
+
+        Assert.False(queue.IsItemPlaying());
+        Assert.Equal(Guid.Empty, queue.GetPlayingItemPlaylistId());
+    }
+
+    [Fact]
+    public void SetPlayingItemByIndex_InBounds_SetsPlayingItem()
+    {
+        var queue = CreateQueue(2);
+        var expectedItemId = queue.GetPlaylist()[1].ItemId;
+
+        queue.SetPlayingItemByIndex(1);
+
+        Assert.True(queue.IsItemPlaying());
+        Assert.Equal(expectedItemId, queue.GetPlayingItemId());
+    }
+}